Wave signaling method and apparatus



July 9, 1940- R. A. HILFERTY WAVE SIGNALING METHOD AND APPARATUS FiledJuly 1, 1939 INVENTOR 22%/ RNY Patented July 9, 94

UNITED STATES WAVE SIGNALING METHOD AND APPARATUS Richard A. Hilferty,Oceanside, N. Y., assignor to Press Wireless, Inc., Chicago, Ill., acorporation of Delaware Application July 1 1939,v Serial No. 282,356v

12 Claims.

This invention relates to wave signaling systems and more especially tomethods and apparatus for compensating for level variations in atransmitting medium of the wire or wireless type.

A principal object of the invention is `to provide an improved method oftransmitting signals, such for example as tele-facsimile signals,telegraph signals and the like, so that the selective y fadingcharacteristic of the transmitting me- 10 dium are compensated for.

Another object is to provide a system of telefacsimile transmissionwherein the facsimile signals are simultaneously transmitted on a bandvof heterogeneous carrier or sub-carrier frequen- Cil l5 cies havingpredetermined lower and upper frequency limits.

Another object is to provide a system of auto- `matic gain control for areceiving amplifier which utilizes the average amplitude of a receivedheterogeneous frequency band for level control, thus avoiding thedrawbacks incident to the use of a single level control frequency. Inaccordance with this object, instead of transmitting the signals as amodulation of a single fundamental frequency carrier or as a modulationof a plurality of simultaneous carriers of one or more dominant orfundamental frequencies, the signals are transmitted as modulations of anoise band carrier, that is one wherein a large number of frequenciesare generated but without any dominant frequency. An example of such aheterogeneous frequency band may be mentioned the noise currents'whichare generated when an oscillating circuit is suddenly quenched at alower rfrequency, of which a super-regenerative oscillao tor of anyknown type is typical. I have found that this ordinarily undesirablenoise or hissing can be practically utilized as a carrier or subcarrierfor signals such as facsimile signalsor the like, and that it has highlyadvantageous results when used for automatic level control at areceiving station.

Accordingly a feature of the invention relates to a signaling systemwherein a noise gen-l erator of a heterogeneous frequency band isemployed as a carrier or sub-carrier of message signals such astelegraph signals, facsimile signals and the like.

Another feature relates to a signal transmis- -0 sion system wherein aheterogeneous carrier frequency band is employed so as to produce asmooth noise or hiss over a relatively wide portion of theaudio-frequency spectrum, the amplitude ofthe band being modulatedin'accord- ,55 ance with signals to be transmitted. I

(Cl. Z50-6) tion of the following detailed descriptions andl theappended claims. 10

inasmuch as the invention in its broad aspects is concerned with themethod and apparatus of generating and utilizing a carrier orsub-carrier of the heterogeneous frequency band type, only those partsof a signaling system are shown l5 as will enable the inventive conceptto be understood and practised. Accordingly, apparatus land connectionswhich are old or well-known in the art will be illustrated in thedrawing in simpliiied or diagrammatic form. 20

Fig. l shows in schematic form a transmission system embodying featuresof the invention.

Fig. 2 is `a modification of Fig. l.

i Fig. 3 isa more detailed diagram of` connections of the system of Fig.2 showing the hetero- '25 geneous frequency generator that may also beused in the system of Fig. l.

ylt can be shown by mathematical analysis that noise generated by theexcitation or quenching 'of anoscillating circuit by means of other 30waves of steep wave front, consists of innumerable frequency components.Thus where the excitation or quenching is by means of a square wavepulse, the frequencies .are theoretically .infinite in number. It isknown that by using a grid-controlled vacuum tube and exciting it withsquare iront waves, that .-a series of separate frequencies are producedin the output f circuit, however these frequencies are generated on aregurla'rly continuous time basis'and successively higher frequenciesare regularly recurrent as far..A as the time axis is concerned. Such anarrangement is quite commonly used in carrier telephone systems. Inoontradistinction to this known type of harmonic or homogeneousfrequency generator, the present invention contemplates the generationand utilization of the innumerable component frequencies which aregenerated ona c. random time axis and willbe referred to herein as aheterogeneous frequency band generator. 50 It is possible by properexcitation of a grid-controlled vacuum tube yto generate such hetero-`geneous frequency rbands so that the recurrence of the individualfrequencies of the band is not on a harmonic basis, and ,the frequencyrecurrence of any given Wave or waves is fortuitous. When suchheterogeneous frequencies are generated in the audio frequency band theyare evident to the ear as a smooth hissing in which no particularfrequency is outstanding or dominant. In accordance with the presentinvention, a predetermined portion ofthe spectrum of such aheterogeneous frequency generator is selected for example by bandfilters or the like, and this band is used Yas a carrier or sub-carrierof signals to be transmitted over a wire channel or a wireless channel.Consequently since at any given instant at the receiver a signal isrepresented by an innumerable number of individual heterogeneous carrieror sub-carrier frequencies, the effects of selective fading can bematerially reduced at the receiver. One manner of utilizing thisheterogeneous frequency band is to apply it to control the generationofan automatic volume control signal for application to a receivingamplifier. Thus where the signals are transmitted over a radio channel,it is well-known that individual frequencies are very often subjected toselective or non-uniform fading. Consequently two frequencies separatedby a small fraction of a percent will not always fade simultaneously andto the same extent. In the usual system of amplitude-modulated carriertransmission Where the transmission is carried out by means of asubcarrier usually of audio frequency, only a single sub-carrier isemployed for the signals. Consequenly at any given instant at thereceiver, there will be present the sub-carrier frequency and its upperand lower side-band frequencies. It is the usual practice in applying A.V. C. signals at the receiver, to generate such A. V. C. signals inaccordance with the received carrier or sub-carrier frequency alone, andthe side-band fequencies may fade in and out irregularly with respect tothe carrier or subi-carrier although the latter may be held quiteconstant. In accordance with the present invention, by using sub-vcarrier frequencies of the heterogeneous frequency band type andutilizing these hetero-- geneous frequencies for A. V. C. controlpurposes there is assured at all times a considerable number offrequencies for the A. V. C. control. Consequently even if somefrequencies are actually missing as a result of selective fading, therewill be sufficient remaining to prevent a complete loss of the receiveroutput.

vReferring to Fig. l, the numeral I represents a device for generating aheterogeneous frequency wave spectrum wherein the individual'frequenciesare generated fortuitously or at random rso far as timing is concerned.For example there is shown in Fig. 3, one typical generator comprising agrid-controlled vacuum tube 2 having its input and output circuitsconnected in feedback relation through the tuned oscillatory circuit 3,4, so as to generate super-audible oscillations which are interrupted ata super-audible rate under control of the grid condenser 5 and grid leakresistance 6, it being understood that the D. C'. voltage for plate 'lis applied through the output transformer winding 3 in series with thehigh frequency choke coil 9. This generator is therefore of thewell-known super-regenerative type and any other well-known form of suchgenerator may be employed. As is well-known in the art, the output ofsuch a generator produces a steady hissing or smooth noise which is freefrom any one dominant or regularly recurrent frequency component.

The audio-frequency or hissing output of device I (Fig. 1) is thenpassed through a suitable` band-pass filter l!) of known design wherebya predetermined section of the audio-frequency spectrum of the noisecurrents is passed to Device Il may means for optically scanning arepresentation to convert it into corresponding electric signals. Theoutput of filter lll is applied to the facsimile scanning equipment sothat the output of device il consists of the heterogeneous frequencysub-carrier of audio-frequency modu-V lated in accordance with theinstantaneous facsimile signals. The modulated heterogeneous frequencysub-carrier can then be amplified in any suitable amplier l2 andimpressed upon the transmission link i3 which may be a wire chan nel ora radio channel.

At the receiving station, to whichl the channel i3 is connected, thereceived waves are amplified-inv a suitable amplier itlincludingpreferably a stage employing a grid-controlled arnplifier tube(not shown) which is arranged in the usual way to have its gaincontrolled b-y an A. V. C. signal. After amplification the heterogeneousfrequency sub-carrier is. detected in any Well-known detector wherebythe original signals are also detected and applied to theaudio-frequency amplier I5 the output of which is applied to anywell-known form vof signal repro- A portion of the detected sub-carrierducer lll. heterogeneous frequencies from device l5 are applied to anywell-known form` of A. V. C.l circuit !8 whereby there is produced a D.C. signal representative of the average amplitude of the heterogeneouscarrier frequencies as actually reamplified in amplifier 2l and areapplied to any" well-known form of modulator network 22 which is fedwith a heterogeneous frequency yband sub-carrier from the generator 23through the band-pass filter 2f. After amplification in theaudio-frequency amplifier 25 the signal modulated sub-carrier is appliedto the modulationnetwork 26 of any Well-known form of carriertransmitter the carrier generator of which is represented by the device2l. The doubly modulated carrier wave is then transmitted to thereceiving station Where after suitable amplification it is subjected todouble demodulation to detect the heterogeneous frequency sub-carrier inorder to generate the local A. V. C. signal as described in connectionwith Figi, and to detect from the sub-carrier the original signals to bereproduced.

Referring to Fig. 3, there `is shown. in more detail, one modulating.arrangement that is represented by the blocks 22, 23, 25 of' Fig. 2.The heterogeneous frequency generator 23 ris represented by thegrid-controlled vacuum tube l which as above described is provided withknown circuit connections whereby it acts as a superregenerativeoscillator.

Connected in the gridv cathode circuit of the generator is a grid-biaswresistor to which is applied the output of the signal amplifier 2| sothat the noise current in the output transformer 29 is modulated inaccordance with signals from source 20. It will be understood of coursethat any other suitable way of modulating the noise currents of thesuper-regenerative oscillator to produce the signal-modulatedheterogeneous frequency sub-carrier may be employed, and other changesand modifications may be made in the disclosed embodiments withoutdeparting from the spirit and scope of the invention. While in theforegoing description reference has been made to the production of aheterogeneous frequency in the audio frequency band, it will beunderstood of course that the heterogeneous frequencies thatare used asthe carrier or sub-carrier may be in the superaudible spectrum or in theradio frequency spectum.

What I claim is:

l. The method of signaling which includes the steps of generating aheterogeneous frequency band wherein the frequency recurrence of anygiven wave or waves is fortuitous, modulating said band by a signal tobe transmitted, and applying the modulated band to a transmissionchannel.

2. The method of signaling which includes the steps of generating aheterogeneous frequency band whereinthe frequency recurrence of anygiven wave or waves is fortuitous, modulating said band by a signal tobe transmitted, receiving the modulated band, and detecting from thereceived band the signal to be transmitted.

3. The method of signaling which includes the steps of generating anoise band consisting of heterogeneous frequencies wherein the frequencyrecurrence of any given wave or waves is fortuitous, modulating saidband by a signal to be transmitted, receiving the modulated band toAde-v tect the signal therefrom, and deriving from the receivedheterogeneous frequencies a level con-` trol signal.

4. The method of signaling which includes the steps of generating acarrier in the form of a band of heterogeneous audio frequencies whereinthe frequency recurrence of any given wave or waves is fortuitous,modulating said band by a signal to be transmitted, receiving anddetecting the lsignal from the band, and controlling the level of thedetected signal by generating a levelv ing a portion of the receivedband to control the level of the amplification to compensate forselective fading in the transmission medium.

6. The method of radio signaling which includes the steps of generatinga plurality of subcarrier frequencies of heterogeneous frequencieswherein the frequency recurrence of any given wave or waves isfortuitous, modulating said frequencies by a signal to be transmitted,modulating a radio carrier by said modulated frequencies, receiving themodulated radio carrier and detecting therefrom the sub-carriers, anddetecting from the sub-carriers said signal.

7. The method of signaling employing a superregenerative oscillatorwhich includes the steps of generating a signal to be transmitted,exciting the oscillator to produce heterogeneous waves wherein thefrequency recurrence of any given wave or waves is fortuitous, applyingsaid signal to the oscillator to modulate the normal noise currents inthe output circuit thereof, and transmitting said modulated noisecurrents to a distant receiving station.

8. In a signaling system, means to generate a band of heterogeneousfrequencies wherein the frequency recurrence of any given wave or Wavesis fortuitous, means to modulate said band by a signal to betransmitted, and means to apply said modulated band to a transmissionchannel.

9. In a. signaling system, means to generate a series of heterogeneousfrequencies wherein the frequency recurrence of any given Wave or wavesis fortuitous, the series being free from any dominant or fundamentalfrequency, means to modulate all said frequencies simultaneously by asig-` nal, and means to transmit said modulated frequencies to a distantreceiving station.

10. In a signaling system, a super-regenerative oscillator forgenerating noise currents composed of heterogeneous frequencies whereinthe frequency recurrence of any given wave or waves is fortuitous, aband-pass lter for selecting a. predetermined band of said frequencies,and means to modulate said selected band by a signal to be transmitted.

11. A signaling system according to claim 10, in which means areprovided to receive said modulated band and to detect therefrom thesignal being transmitted, and means to derive from the received -noisecurrents a level control signal.

12. In combination a grid-controlled vacuum tube, a feed-back circuitbetween the input and outputof said tube for generating super-audibleoscillations, means associated with said tube to interrupt saidoscillations to generate in the output circuit noise currents whereinthe frequency recurrence of any given wave or waves is fortuitous, meansto apply a signal to be transmitted to said tube to modulate said noisecurrents, and means to apply said modulated noise 00 currents as acarrier to a transmission channel.

RICHARD A. I-IILFERTY.

